1. Field of the Invention
The present invention generally relates to semiconductor devices and methods for fabricating the same. More particularly, the present invention generally relates to semiconductor devices having a metal-oxide semiconductor (MOS) varactor and methods for fabricating the same.
A claim for priority is made to Korean Patent Application No. 2004-9380, filed Feb. 12, 2004, the contents of which are hereby incorporated by reference in their entirety.
2. Description of the Related Art
A varactor is a voltage-variable capacitor used in analog integrated circuit such as a voltage controlled oscillator (VCO) which is one of the most important circuit blocks for radio frequency (RF) front-end. The varactor requires a wide tuning range and a high Q value.
The varactor integrated in a semiconductor device is generally a junction varactor or a MOS varactor. The tuning range in a junction varactor is usually narrower than that of a MOS varactor and does not improve preferably as a CMOS device scales down. Hence, as semiconductor devices become highly integrated, the MOS varactor is more likely becoming a choice in a high frequency integrated circuit.
U.S. Pat. No. 5,038,184 discloses a conventional method of fabricating a MOS varactor.
Generally, the MOS varactor includes an active semiconductor layer and a gate electrode. A capacitor dielectric layer is interposed between the active semiconductor layer and the gate electrode. Therefore, when voltage is applied to the gate electrode, electrical charge accumulates on or depletes from the active semiconductor layer, and accordingly varies the capacitance of the varactor. In case of an accumulation mode MOS varactor, as electrical charge accumulates in the active semiconductor layer, the varactor reaches a maximum capacitance “Cmax”. As electrical charge is fully depleted from the active semiconductor layer, the varactor reaches a minimum capacitance “Cmin”. The ratio of “Cmax” to “Cmin” determines the tuning range.
The conventional varactor generally employs poly silicon as a gate electrode. Therefore, as electrical charge accumulates in the active semiconductor layer, poly depletion occurs in the gate electrode. The poly depletion in the gate electrode lowers the value of “Cmax”, which decreases the tuning range of the varactor. To increase the tuning range of the varactor, the poly depletion must be suppressed in the gate electrode.
In addition, semiconductor integrated circuits require various resistors. In particular, a high frequency integrated circuit requires a reliable metal resistor. The metal resistor is fabricated by forming and patterning a metal film. A photomask is required to pattern the metal film. The use of an additional photomask increases the manufacturing cost of the semiconductor device.